A hand-held electrically powered cut-off tool with a kickback mitigation function

ABSTRACT

A hand-held electrically powered cut-off tool (100) for cutting concrete and stone by a rotatable cutting disc (105), the cut-off tool (100) comprising an electric motor (130) arranged to be controlled by a control unit (110) via a motor control interface (120), wherein the control unit (110) is arranged to obtain data indicative of an angular velocity of the cutting disc (105), and to detect a kickback condition based on a decrease in angular velocity, wherein the control unit (110) is arranged to control an electromagnetic braking of the electric motor (130) in response to detecting a kickback condition, wherein the control unit (110) is arranged to determine an angular acceleration associated with the electric motor (130), and to detect the kickback condition based on a comparison between the determined angular acceleration and a configurable detection threshold.

TECHNICAL FIELD

The present disclosure relates to electrically powered hand-held cut-offtools for cutting concrete and stone, and in particular to kickbackmitigation systems specifically tailored for such tools.

BACKGROUND

Cut-off tools for processing hard materials such as concrete and stonerequire powerful motors which provide the necessary energy to processthe hard materials. Electrically powered cut-off tools have recentlybeen introduced. These machines comprise high performance batterieswhich power high torque electric motors. Some electrically poweredcut-off tools are also powered via cable from electrical mains.

On rare occasions, the rotating cutting disc of the cut-off tool entersinto locking contact with the object that is processed. Due to the largeamounts of kinetic energy stored in the rapidly rotating cutting disc,the disc will be ejected from the object and the cut-off tool will moveupwards and backwards towards the operator. This is referred to as akickback condition, and it may cause severe injury to the operator. Itis therefore highly desirable to avoid kickback events, and to mitigatethe effects of a kickback event if it should anyway occur.

U.S. Pat. No. 10,675,694 B2 discloses a braking device able to quicklystop rotation of a rotating cutting disc with high inertia. A brake unitacts on a belt drive of a cut-off tool to efficiently brake a cuttingdisc from a state of high kinetic energy.

U.S. Pat. No. 8,413,340 B2 discloses a safety guard assembly formitigating the harmful effects of a kickback event. The assemblyincludes a safety guard, a locking mechanism, and a weight. In akick-back situation, the locking mechanism quickly releases, and theweight forces the guard to swing rapidly down over the guard, therebyproviding protection from the saw blade.

US 2011/0007436 A1 discloses devices and methods for safety precautionin electrical tools. The disclosure comprises a sensor unit which isdesigned to generate a sensor signal based on, e.g., a motor current ofan electric motor. The sensor signal can be used to trigger an electricbraking action by the electric motor.

EP 3 260 242 A1 and US 2020/0206887 A1 also relate to safety mechanismsfor use with electrical powertools, which mechanisms comprise detectionof a kickback event based on a change in rotation of a rotatable tool,followed by a braking action to slow down the tool.

However, there is a continuing need for improved kickback mitigationsystem.

SUMMARY

It is an object of the present disclosure to provide electricallypowered hand-held cut-off tools with improved kickback mitigationsystems. This object is obtained by a hand-held electrically poweredcut-off tool for cutting concrete and stone by a rotatable cutting disc.The cut-off tool comprises an electric motor arranged to be controlledby a control unit via a motor control interface. The control unit isarranged to obtain data indicative of an angular velocity of the cuttingdisc, and to detect a kickback condition based on an abrupt decrease inangular velocity. The control unit is also arranged to control anelectromagnetic braking of the electric motor in response to detecting akickback condition, and preferably also to actively regulate an energyouttake from the electric motor over the control interface during theelectromagnetic braking.

This way a kickback condition can be detected very rapidly by thecontrol unit, and the electromagnetic braking can be made powerfulenough such that the kickback event can be stopped well before the eventbecomes dangerous to an operator of the cut-off tool. In fact, in mostcases the kickback mitigation systems discussed herein are able to haltthe kickback event before the cutting disc even leaves the object whichis being cut. The braking operation is preferably a controlled brakingoperation which is regulated by the control unit. This controlled energyouttake from the electric motor reduces the risk for component damageand the like, while still providing efficient kickback mitigation.

According to aspects, the control unit is arranged to estimate a rotorangle of the electric motor based on a measured current over the controlinterface, and to obtain the data indicative of angular velocity as adifference of the rotor angle over time. Several methods for estimatingrotor angle based on measurements of current over the control interfacebetween control unit and electric motor are known. These methodsadvantageously do not require external sensors for measuring rotorangle. For instance, the control unit can be arranged to determine anangular position of a rotor of the electric motor based on dataindicative of a rotor flux angle of the electric motor, and to obtainthe data indicative of angular velocity as a difference of the rotorangular position over time. Since the rotor angle determination can bedone without signals from external sensors, the entire kickbackmitigation system disclosed herein can be integrated in its entirety inthe control unit and electric motor assembly, which is an advantage.Advantageously, there is no need for advanced braking devices such asthat disclosed in U.S. Pat. No. 10,675,694 B2.

According to aspects, the hand-held electrically powered cut-off toolcomprises an energy dissipating module configured to dissipate energyfrom the electric motor during the electromagnetic braking in acontrolled manner. This energy dissipating module can be used by thecontrol unit to perform braking in a controlled manner without risking,e.g., too high energy levels in the control unit circuitry, in theelectric machine windings, or on the control interface. The energydissipating module may, for instance, comprise any of a resistance, asuper-capacitor, a cable connection to electrical mains and/or a batteryconfigured with an energy absorption capacity.

According to aspects, the control unit is arranged to obtain the dataindicative of the rotor flux angle of the electric motor based on ameasured current over the control interface. This measurement of currentis not associated with any significant implementation complexity in thecontrol unit, which is an advantage. However, the control unit may alsobe arranged to obtain the data indicative of a rotational velocity ofthe cutting disc at least in part from an external sensor, such as aHall effect sensor or the like configured to measure rotations of theelectric motor shaft. The external sensor can be used as an alternativeto the current measurements on the control interface, or in combinationwith the current measurements on the control interface for increasedreliability.

According to aspects, the control unit is arranged to process the dataindicative of the angular velocity of the cutting disc by a firstlow-pass filter and by a second low-pass filter, where the firstlow-pass filter has a larger bandwidth compared to the second low-passfilter. The first low-pass filter is applied for kickback eventdetection, and the second low-pass filter is applied otherwise.

This way the normal electric motor control is associated with a largernoise suppression since a lower bandwidth low-pass filter is used. Thekickback event detection is preferably more rapid, which is why thehigher bandwidth filter is used. Thus a robust motor control is providedwhile at the same time a rapid kickback detection is enabled.

According to some aspects, the speed regulator function is bypassed whenthe control unit controls the electromagnetic braking during a kickbackevent to enable a more rapid braking operation.

According to aspects, the control unit is arranged to determine anangular acceleration associated with the electric motor, and to detectthe kickback condition based on a comparison between the determinedangular acceleration and a detection threshold. This is a relatively lowcomplexity detection principle which nevertheless provides robustdetection performance associated with a high detection probability and alow probability of false alarm.

According to aspects, the control unit is arranged to detect thekickback condition also based on an angular velocity associated with theelectric motor by conditioning kickback detection based on a minimumangular velocity. This way false detections during, e.g., tool startfrom standstill is avoided, which is an advantage. The detectionthreshold can be manually configurable or arranged to be automaticallyconfigured by the control unit, e.g., in dependence of tool inertia, orconfigured via wired or wireless link from a remote device, such as asmart phone or a remote server. Thus, an operator can use a display onthe device to set the threshold to some desired value, i.e., if thekickback detection mechanism is experienced as too sensitive, then theoperator can adjust the detection threshold to obtain a more desirablebehavior. A remote operator can also configure the threshold, e.g., aspart of a software upgrade, or in case an operator submits a request forreconfiguration of the threshold. The configuration can of course alsobe made from a wireless device, such as a smart phone or tablet device.The minimum angular velocity may be on the order of 400-800 rad/s, suchas about 600 rad/s.

According to aspects, the control unit is arranged to obtain dataindicative of a tool diameter or indicative of a tool inertia of therotatable cutting disc, and to adjust the detection threshold based onthe data indicative of tool diameter or tool inertia. This way kickbackdetection can be optimized to suit operation with a given tool. Somemore heavy cutting discs may be associated with slightly differentbehavior in terms of decreased acceleration during kickback eventscompared to lighter cutting discs. If the tool inertia is approximatelyknown, such differences can be compensated for in order to obtain a morereliable and accurate kickback detection mechanism. The control unit mayfor instance be arranged to obtain the data indicative of the tooldiameter or tool inertia from manual input, or based on a calculated oran estimated tool inertia, wherein the tool inertia is arranged to bedetermined based on a current drawn by the electric motor duringacceleration, e.g., from a standstill or low velocity condition. Thus, arobust mechanism for estimating tool inertia without external sensors orthe like is provided. This mechanism can be applied to any cutting discattached to the tool which is an advantage.

According to aspects, the cut-off tool comprises a radio frequencyidentification (RFID) reader, and the control unit is arranged to obtainthe data indicative of the tool diameter or tool inertia from an RFIDdevice embedded into the cutting disc via the RFID reader. This way thetool diameter or tool inertia data is obtained directly from the cuttingdisc in a reliable manner. When the cutting disc is replaced by anothercutting disc, the data is automatically updated.

According to aspects, the cut-off tool comprises means for detectingtool identification data (ID) from an optically readable tag arranged onthe tool or on a tool packaging associated with the tool, and to obtainthe data indicative of the tool diameter or tool inertia based on thetool ID. The cut-off tool may, for instance, be configured to contact aremote server or the like to obtain the required cutting disc data. Thecall to the remote server may comprise the tool ID obtained from theoptically readable tag. The cut-off tool may comprise a radiotransceiver, and the control unit can be arranged to obtain the dataindicative of the tool diameter or tool inertia from the remote servervia the radio transceiver.

According to aspects, the control unit is configured to control theelectromagnetic braking of the electric motor by applying a configurablebraking torque in dependence of a pre-determined time limit for brakingthe rotatable cutting disc. This means that the control unit does notalways need to apply maximum braking force, and thereby spare componentssuch as a braking resistor and other electrical components fromincreased wear due to hard use.

According to aspects, the control unit is configured to control theelectromagnetic braking of the electric motor to generate a torquedetermined in dependence of a direct current (DC) bus voltage of thecut-off tool. This is a relatively simple method for controlling theapplied torque. It also protects large parts of the control unitcircuitry from dangerously high DC levels in an efficient manner.

According to aspects, the control unit is configured to control theelectromagnetic braking of the electric motor to generate a torquedetermined in dependence of an energy dissipating capability of theenergy dissipating module. The energy dissipating module is associatedwith a maximum energy dissipating capability, i.e., a maximum amountwhich can be dissipated over a period of time. By generating brakingtorque in dependence of this energy dissipating capability, overloadingthe energy dissipating module can be prevented.

According to aspects, a DC bus voltage of the cut-off tool is regulatedby switching a device associated with an impedance duringelectromagnetic braking of the electric motor. This switching mechanismprovides a relatively simple yet reliable mechanism for regulating theDC voltage.

According to aspects, the control unit is configured to control theelectromagnetic braking of the electric motor to generate a brakingtorque below a maximum braking torque level associated with a maximumrate of change in motor shaft angular speed. If the electric motor isbraked too rapidly, the estimate of rotor angle may suffer in accuracy,which in turn will degrade braking capability. By generating a brakingtorque below the maximum braking torque level, the estimate of rotorangle can be kept at an accurate level such that the braking performanceis not reduced.

According to aspects, the cut-off tool comprises a support arm and therotatable cutting disc is arranged to be driven by the electric motorvia a belt drive and a geared transmission. This allows for a gear ratiowhich reduces the torque requirements on the electric motor and controlunit assembly during the kickback mitigation braking.

Generally, all terms used in the claims are to be interpreted accordingto their ordinary meaning in the technical field, unless explicitlydefined otherwise herein. All references to “a/an/the element,apparatus, component, means, step, etc.” are to be interpreted openly asreferring to at least one instance of the element, apparatus, component,means, step, etc., unless explicitly stated otherwise. The steps of anymethod disclosed herein do not have to be performed in the exact orderdisclosed, unless explicitly stated. Further features of, and advantageswith, the present invention will become apparent when studying theappended claims and the following description. The skilled personrealizes that different features of the present invention may becombined to create embodiments other than those described in thefollowing, without departing from the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will now be described in more detail withreference to the appended drawings, where

FIG. 1 shows an example of an electrically powered cut-off tool;

FIG. 2 schematically illustrates a general electric motor controlsystem;

FIG. 3 schematically illustrates a three-phase electric motor controlsystem based on an inverter;

FIG. 4 is a functional view of an example kickback mitigation system;

FIG. 5 schematically illustrates a kickback detection system;

FIG. 6 shows a cut-off tool support arm for a circular cutting blade;

FIG. 7 is a cross-sectional view of a support arm for a circular cuttingblade;

FIG. 8 illustrates a drive arrangement for a circular cutting blade;

FIG. 9 is a flow chart illustrating methods;

FIG. 10 schematically illustrates a control unit; and

FIG. 11 schematically illustrates a computer program product.

DETAILED DESCRIPTION

The invention will now be described more fully hereinafter withreference to the accompanying drawings, in which certain aspects of theinvention are shown. This invention may, however, be embodied in manydifferent forms and should not be construed as limited to theembodiments and aspects set forth herein; rather, these embodiments areprovided by way of example so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art. Like numbers refer to like elements throughout thedescription.

It is to be understood that the present invention is not limited to theembodiments described herein and illustrated in the drawings; rather,the skilled person will recognize that many changes and modificationsmay be made within the scope of the appended claims.

FIG. 1 shows a hand-held electrically powered cut-off tool 100 forcutting into hard materials such as concrete and stone. The tool 100comprises a rotatable circular cutting disc 105, which may also bereferred to as a cutting blade, mounted on a support arm 150. Thecutting disc 105 is normally brought into rotation in a down-cutdirection R, i.e., downwards into the object to be cut. The cutting disc105 is arranged for abrasive operation, by means of cutting segments onthe periphery of the cutting disc 105, where the cutting segmentscomprise diamond granules or the like.

An electric motor 130 is arranged to drive the cutting tool via a drivearrangement in the support arm. This motor is powered from an electricalenergy storage device 140, such as a battery or a super-capacitor.Alternatively, the electric motor may be powered from electrical mainsvia cable. An example drive arrangement for driving the cutting disc 105by the electric motor 130 will be discussed in more detail below inconnection to FIGS. 6-8 . This drive arrangement is based on acombination of gears and a belt to reduce requirements on the strengthof the belt and also to reduce the requirements on electric motortorque. However, normal belt drives with two pulleys and an endless beltcan of course also be used.

With reference again to FIG. 1 , the electric motor is controlled by acontrol unit 110 via a motor interface 120. This is also schematicallyshown in FIG. 2 and in FIG. 3 . The motor interface may vary in functionand physical realization, but the control unit 110 controls electricmotor speed over the interface, and may both accelerate and decelerate,i.e., brake, the electric motor via the motor interface 120.

The machine body 101 may comprise a display unit configured to displayvarious status messages to an operator of the tool 100. This displayunit may also comprise input means, which the operator can use toconfigure various parameters of the device. For instance, the operatorcan use the input means to select or otherwise configure a sensitivitylevel of a kickback detection mechanism, which will be discussed in moredetail below.

The tool 100 may also comprise a transceiver arranged to communicatewith a remote device, such as a remote wireless device. This device canthen also be used to configure one or more parameters on the tool, suchas the sensitivity of a kickback detection and mitigation function.

The motor is preferably a permanent magnet synchronous motor (PMSM)which is an alternating current (AC) synchronous motor whose fieldexcitation is provided by permanent magnets, and which has a sinusoidalcounter-electromotive force (counter EMF) waveform, also known as backelectromotive force (back EMF) waveform. PMSM motors are known ingeneral and will therefore not be discussed in more detail herein. Forinstance, similar electrical motors including associated control methodsare discussed in “Electric Motors and Drives” (Fifth Edition), Elsevier,ISBN 978-0-08-102615-1, 2019, by Austin Hughes and Bill Drury.

The motor 130 may be a three-phase motor as schematically shown in FIG.3 . In this case the motor interface 120 comprises three wires forenergizing the motor windings. The wires are fed from an inverter 115which is normally controlled by a current command from the control unit110. An inverter is a module which generates one or more phases ofalternating current, normally from a DC feed. By controlling thefrequency and voltage of the phases over the motor interface 120, theelectromagnetic field in the motor can be brought into a controlledrotation to generate a positive torque by the motor shaft, which thencan be used to power the cutting disc 105. The electric motor can alsobe used to provide negative torque to the motor shaft, i.e., to brake tocutting disc 105.

The present disclosure relates to a kickback mitigation function forhand-held electrically powered cut-off tools which relies on quicklydetecting onset of a kickback event, followed by a controlled andresolute application of negative torque by the electric motor to rapidlybrake the cutting disc 105. It has long been thought thatelectromagnetic braking cannot be applied fast enough and with enoughforce to mitigate kickback in high power cut-off tools, mainly since thecutting disc stores so much kinetic energy during operation. However, bythe techniques disclosed herein, kickback mitigation in high powercut-off tools is enabled. The techniques do not require a mechanicalbrake, which is an advantage. Another advantage is that there is norequirement of any external sensors to detect the kickback condition,since the control unit can perform reliable detection based solely onthe signals over motor interface 120. However, it is appreciated thatexternal sensors can be used to complement the system in order toprovide an increased level of reliability and robustness. Furthermore,the kickback mitigation mechanism which will be described in thefollowing is so fast that it is often able to stop the kickback beforethe cutting disc even leaves the object, which is being cut, therebypreventing all harmful effects of the kickback event.

Electromagnetic braking of electrically powered hand-held tools has beenproposed before, but for other applications with much less strictrequirements on detection delay and generated braking torque. Forinstance, hammer drills and the like are associated with a significantlysmaller kinetic energy and are thus much more easily braked. U.S. Pat.Nos. 10,675,747 B2 and 7,055,620 B2 show electromagnetic braking systemsfor mitigating effects of stuck drill bits, which are not directlyapplicable for kickback mitigation in high powered cut-off tools.

U.S. Pat. No. 10,630,223 B2 describes another example of a power toolcomprising means to brake a rotatable tool automatically based onelectromagnetic braking principles. The disclosure describes a mechanismfor detecting a kickback condition based on an external sensor, such asa Hall effect sensor.

The sensor is configured to measure a number of rotations of a rotatablework tool. If the number of rotations has decreased significantly fromone time instant to another, then a kickback condition is detected, anda response is triggered. This polling operation is most likely not fastenough to respond to a cut-off tool kickback condition in a timelymanner, i.e., within a few milliseconds from onset of the kickbackcondition. The braking action described in U.S. Pat. No. 10,630,223 B2mainly comprises disconnecting the electric motor from the power sourcein order to avoid damage to a workpiece, which is a rather rudimentaryform of braking not likely to be able to cope with the large amounts ofkinetic energy present in hand-held electrically powered cut-off tools.Also, there is no disclosure of any actively regulated or controlledbraking function. Rather, the braking operation relies on mechanicalswitching of resistances. Once triggered, the counter EMF of the systemdescribed in U.S. Pat. No. 10,630,223 B2 will depend on the angularvelocity of the electric motor shaft, so the applied braking torque willbe a function of the rotational velocity of the tool and cannot becontrolled. To summarize, the mechanisms disclosed in U.S. Pat. No.10,630,223 B2 are not ideal for mitigating kickback effects in highpower cut-off tools where the amount of kinetic energy is very largecompared to other types of power tools such as drills, grinders, andhandheld saws.

To provide a kickback mitigation function which is suitable also forhigh powered cut-off tools associated with significant tool inertia,that responds fast enough and with sufficient braking force, there isdisclosed herein a hand-held electrically powered cut-off tool 100 forcutting concrete and stone by a rotatable cutting disc 105. The cut-offtool 100 comprises an electric motor 130 arranged to be controlled by acontrol unit 110 via a motor control interface 120. The control unit 110is arranged to obtain data indicative of an angular velocity of thecutting disc 105, and to detect a kickback condition based on a decreasein angular velocity. The control unit 110 is arranged to controlelectromagnetic braking of the electric motor 130 in response todetecting a kickback condition, and optionally also to actively regulatean energy outtake from the electric motor 130 over the control interface120 during the electromagnetic braking.

Herein, “indicative of an angular velocity” is to be construed as anydata value or sequence of data values from which an angular velocity ofthe cutting disc can be inferred, at least approximately. Thus, a timesequence of rotation angle values is indicative of angular velocitysince velocity can be obtained from the sequence by differentiation. Atime sequence of acceleration values is also indicative of angularvelocity since angular velocity can be inferred from the accelerationvalues by integration. Thus, it is appreciated that there are many waysto represent data indicative of angular velocity.

In the same way, there are many ways in which a kickback condition canbe detected “based on a decrease in angular velocity”, starting fromdifferent types of data, and using different methods. A number of suchmethods will be discussed herein, although it is appreciated that thisis not to be construed as an exhaustive list of embodiments in which thepresent techniques may be carried out.

The detection mechanism is based on monitoring the angular velocity ofthe cutting disc 105. If an abrupt decrease in velocity is seen, such asa high level of retardation in electric rotor angle or cutting discangle, a kickback condition is detected. The details of the kickbackevent detection mechanism will be discussed in more detail below.Immediately after a kickback event has been detected by the control unit110, the electric motor is forcefully braked in order to mitigate theeffects of the kickback event. This braking involves an active controlof the energy outtake from the electric motor in order to provide astrong braking force without damaging the electrical components of thecut-off tool.

As mentioned above, the kickback detection and braking of the cuttingdisc is often so rapid as to stop the blade before it even leaves theobject which is processed. Thus, the upwards and backwards motion K inFIG. 1 can often be avoided altogether. Even if some kickback motionoccurs, the energy transferred from the cutting disc 105 to the machinebody 101 will be reduced to a level as to mitigate the harmful effectsof the kickback event. Notably, the electric motor is not justdisconnected from the power source 140 as in many of the prior artdocuments. Rather, the energy outtake from the electric motor isactively regulated to provide a strong enough braking action to halt thekickback event.

For instance, according to some aspects, the current from the motor isregulated actively during braking, and it can therefore be maximizedindependently of electric motor angular speed. This means that brakingcan be controlled during the braking process to always maintain strongbraking effect. If a resistance is simply switched in as in some of theprior art, the counter-EMF of the electric motor will determine thebraking force, thus leaving much room for improvement in brakingcapability.

The limiting factor in providing a strong braking torque is the energydissipating capability of the electrical system. To brake the cuttingdisc 105, its kinetic energy must be transferred away from the cuttingdisc and dissipated by the system. The energy transfer must also be fastenough since otherwise the kinetic energy is transferred into themachine body 101 to generate the kickback movement K. This energytransfer is done electrically in the proposed design. Thus, no frictionbrakes or other complicated mechanical structures are required toprovide the necessary braking torque.

FIG. 4 shows a functional view of an example kickback mitigation system.A speed command is obtained, e.g., from the trigger 160 of the tool 100.This speed command is input to a processor 410 which will be discussedin more detail below in connection to FIG. 10 . The processor 410converts the speed command into a current command which is sent to aninverter 420, which in turn controls the electric motor 130 via themotor interface 120. Generally, for most electric machines, a currentcommand is sent to a current controller. The current controller thenoutputs a voltage command which is converted into duty cycles. The dutycycles are then set in the inverter hardware.

In case the motor 130 is a three-phase motor, the control interface 120comprises three wires with respective phases. An energy dissipator 430is connected to the inverter 420. This dissipator is configured toconsume surplus energy in the system, i.e., to dissipate energy from theelectric motor 130 during the electromagnetic braking, therebyprotecting electrical components and the motor 130 itself fromdangerously high voltages. The energy dissipating module 430 maycomprise any of a resistance, a super-capacitor and/or a batteryconfigured with an energy absorption capacity. According to someaspects, the dissipator module 430 is a resistance configured to beswitched by the processor 410 in dependence of a DC voltage of a DC buswhich feeds the inverter with power. This way the DC voltage level onthe DC bus can be regulated to always be close to a target level, orset-point level, despite a large surge of energy coming from theelectric motor 130 via the motor interface 120 during the hard brakingrequired to mitigate a detected kickback event.

It is appreciated that the energy dissipating module 430 may be realizedindependently from the other aspects of the cut-off tools discussedherein. For instance, there is disclosed herein a hand-held electricallypowered cut-off tool 100 for cutting concrete and stone by a rotatablecutting disc 105. The cut-off tool 100 comprises an electric motor 130arranged to be controlled by a control unit 110 via a motor controlinterface 120. The control unit 110 is arranged to obtain dataindicative of an angular velocity of the cutting disc 105, and to detecta kickback condition based on a decrease in angular velocity. Thecontrol unit 110 is arranged to control an electromagnetic braking ofthe electric motor 130 in response to detecting a kickback condition,and the electrically powered cut-off tool 100 comprises an energydissipating module 430 configured to dissipate energy from the electricmotor 130 during the electromagnetic braking.

According to a first example, the resistance is switched in if the DCbus voltage exceeds a first threshold and is switched out if the DC busvoltage goes below a second threshold. The first threshold is preferablyconfigured higher than the second threshold, which effectively meansthat the switching mechanism is associated with hysteresis. Thishysteresis provides a robust detection mechanism.

According to another example, a regulator such as a PID regulator isarranged with a set-point or target DC bus voltage value. This target DCbus voltage value is compared with the actual DC bus voltage value andthe difference is used to determine a duty cycle for switching theresistance.

According to some aspects, the energy dissipating module 430 is notifiedwhen kickback condition is detected, whereupon the energy dissipatingmodule can prepare to absorb surplus energy before that surplus energyarrives at the DC bus. The energy dissipating module 430 may, e.g.,preemptively switch in the resistance or lower the relevant voltagethresholds or target voltage values for performing the switching. Inthis case it may be advantageous to also disconnect the power source,since otherwise current from the power source may be drawn.

A current measurement taken in connection to the motor interface 120 isfed back to the processor 410, whereby a closed loop motor controlsystem is formed. According to some aspects, the control unit 110 isconfigured to electromagnetically brake the electric motor 130 byapplying a configurable braking torque in dependence of a time limit forbraking the rotatable cutting disc 105. This means that the appliedbraking torque can be configured for a particular tool in order tomitigate kickback events. Some tools may require more torque in order tobe braked fast enough while other cutting tools may require less force.Thus, the risk to the electric motor from braking too hard or to otherelectric components can be reduced.

According to other aspects, the control unit 110 is configured toelectromagnetically brake the electric motor 130 at a torque determinedin dependence of a DC bus voltage of the cut-off tool 100. This wayoverload to the DC bus can be avoided, which is an advantage. Forinstance, the DC bus voltage can be regulated by switching a deviceassociated with an impedance, such as a resistor, during electromagneticbraking of the electric motor 130.

The processor maintains an estimate of rotor angle. There are many knownways to estimate rotor angle in an electric machine, e.g., based on thecurrent measurement as schematically illustrated in FIG. 4 . Forinstance, in “Electric Motors and Drives” (Fifth Edition), Elsevier,ISBN 978-0-08-102615-1, 2019, Austin Hughes and Bill Drury discuss thetopic at length.

One example of a method for estimating rotor angle based on a currentmeasurement made on the control interface 120 will now be described. Themethod also uses a reference voltage associated with the electric motor,i.e., the reference voltage upon which the current regulator mechanismis based. It is assumed that the reference voltage (such as thereference voltage used by the current regulator for the electric motor)is sufficiently similar to the actual average voltage over the phases ofthe electric motor over a time window of interest. To clarify, thecurrent regulator in the system generates the reference voltage toregulate the current. However, this can be implemented in a number ofdifferent ways, known in the art.

The reference voltage and control interface current are firsttransformed into a complex stationary domain, i.e., the motor currenti_(ab) and motor reference voltage v_(ab) are represented as complexnumbers. This is often referred to as a Clarkes transform.

i _(ab) =i _(a) +j*i _(b)

v _(ab) =v _(a) +j*v _(b)

Based on these vectors, the complex valued magnetic flux of the stator

ψ_(s,ab)=ψ_(s,z) +j*Ψ _(s,b)

is estimated by integrating a difference between applied voltage andresistive voltage drop, adjusted by a damping factor which isproportional to a previously estimated stator flux. The damping factoris added mainly to make the estimated rotor angle value more robust.

Given the stator magnetic flux, a winding-induced flux is subtracted(derived based on a product of motor winding inductance and motorcurrent) in order to obtain the complex rotor magnetic flux

ψ_(r,ab)=ψ_(r,a) +j*ψ _(r,b)

Thus, let R represent motor resistance and L represent motor inductance,then the electric motor equations in the complex ab-plane (after Clarkestransform) are given by

$\begin{matrix}{v_{a} = {{R*i_{a}} + \frac{d\psi_{s,a}}{dt}}} \\{v_{b} = {{R*i_{b}} + \frac{d\psi_{s,b}}{dt}}}\end{matrix}$

which can be rewritten as

$\begin{matrix}{\frac{d\psi_{s,a}}{dt} = {v_{a} - {R*i_{a}}}} \\{\frac{d\psi_{s,b}}{dt} = {v_{b} - {R*i_{b}}}}\end{matrix}$

These values could be directly integrated to obtain stator flux.However, a damping term is preferably introduced to stabilize theestimated rotor angle. One example of such a damping term operation is

ψ_(s,a) [k]+=(v _(a) [k]−R*i _(a) [k]−K*ψ _(s,z) [k−1])dt

ψ_(s,b) [k]+= 9 v _(b) [k]−R*i _(b) [k]−K*ψ _(s,b) [k−1])dt

where K is a damping factor, K*ψ_(s,z)[k−1] is the damping term referredto above, k is a time index, and dt is a time step of the recursion. Thewinding-induced flux is subtracted as

ψ_(r,a) [k]=ψ _(s,z) [k]−L*i _(a) [k]

ψ_(r,b) [k]=ψ _(s,b) [k]−L*i _(b) [k]

This value is then optionally filtered by a low-pass filter or the liketo suppress noise and distortion. If filtering is applied, then somedelay compensation may be necessary to account for delays introduced bythe filtering and also other delays incurred by, e.g., computation andthe like.

The rotor angle can be found as an angle of the estimated rotor fluxψ_(r,b), i.e., a rotor flux angle. This angle can be determined, e.g.,using a signed arcus tangent function, also known as an atan 2 function

α[k]=atan 2(ψ_(r,b) [k],ψ _(r,a) [k])+β

where α is the rotor angle and where β is an angle compensationconfigured to compensate for introduced delays, e.g., by filteringoperations.

According to another example, the control unit 110 is arranged todetermine an angular position of a rotor of the electric motor, i.e., arotor angle, based on data indicative of a rotor flux angle of theelectric motor, and to obtain the data indicative of angular velocity asa difference of the rotor angular position over time, i.e., a timederivative or time difference value. The control unit 110 may, forinstance, be arranged to obtain the data indicative of the rotor fluxangle of the electric motor based on a measured current over the controlinterface (120), or based on a measured or otherwise determined counterelectromagnetic force (EMF) associated with the electric motor 130.

To improve the estimate of both rotor position and velocity, filteringcan be applied to reduce measurement noise. Such filtering may comprise,e.g., normal low-pass filtering or more advanced filtering techniquessuch as Kalman filtering and the like. However, too much noisesuppressing filtering may increase detection delay which is undesired.

According to some aspects, the control unit 110 is arranged to processthe data indicative of the angular velocity of the cutting disc 105 by afirst low-pass filter and by a second low-pass filter. The firstlow-pass filter has a larger bandwidth compared to the second low-passfilter. The first low-pass filter is applied for kickback eventdetection, and the second low-pass filter is applied otherwise. Thisway, during normal operation, noise suppression is high, but the systemis not able to respond quickly to changes and it would incur too muchdelay in detecting a kickback event. According to an example, theangular position, i.e., the shaft or tool angle, is filtered by a filterto suppress noise and spurious disturbances, from which angular velocityis determined by a difference operation. After the initial filtering,two filters can be arranged to determine velocity, with differentbandwidths. The lower bandwidth filter can then be used with advantagein controlling the electric machine, while the higher bandwidth can beused to detect kickback.

According to other aspects, the control unit 110 is configured toelectromagnetically brake the electric motor 130 at a braking torquebelow a maximum braking torque level associated with a maximum rate ofchange in motor shaft angular speed. This means that the motor brakingwill be done in a controlled manner, which allows, e.g., to maintain anaccurate estimate of the rotor angle in FIG. 4 . If the rate of changein motor shaft angular speed goes above a pre-determined or configurablethreshold, then the braking force can be reduced.

It is appreciated that the arrangements for detecting angular positionand velocity without external sensors can be implemented independentlyof the braking methods used in the tool. Thus, there is also disclosedherein a hand-held electrically powered cut-off tool 100 for cuttingconcrete and stone by a rotatable cutting disc 105. The cut-off tool 100comprises an electric motor 130 arranged to be controlled by a controlunit 110 via a motor control interface 120. The control unit 110 isarranged to determine an angular position of a rotor of the electricmotor 130 based on an estimated rotor flux angle of the electric motor,and to obtain data indicative of an angular velocity of the cutting disc105 based on a rate of change of the angular position of the rotor overtime. The control unit 110 is arranged to detect a kickback conditionbased on a decrease in angular velocity of the cutting disc 105, and toelectromagnetically brake the electric motor 130 in response todetecting a kickback condition.

An example functional view of the kickback detector module 440 is shownin FIG. 5 . This module operates on rotor angle data which is firstdifferentiated once 510 to obtained rotor speed and then again 520 toobtain rotor acceleration. The differentiation is optionally associatedwith a filtering operation to suppress measurement noise. However, it isappreciated that all such noise suppressing filtering increasesdetection delay which is undesirable, thus, a balance should be madebetween detection delay and noise suppression ability in the system, asdiscussed above.

Kickback detection is performed by an evaluation module 530 whichcompares the rotor acceleration to a detection threshold. If asufficiently large negative acceleration is detected, then a kickbackevent is detected, and a brake command is issued by the evaluationmodule 530. In other words, the control unit 110 is arranged todetermine an angular acceleration associated with the electric motor130, and to detect the kickback condition based on a comparison betweenthe determined angular acceleration and a detection threshold.

The aspects related to the implementation of a detection threshold maybe realized independently from the other aspects disclosed herein. Thus,there is disclosed herein a hand-held electrically powered cut-off tool100 for cutting concrete and stone by a rotatable cutting disc 105. Thecut-off tool 100 comprises an electric motor 130 arranged to becontrolled by a control unit 110 via a motor control interface 120. Thecontrol unit 110 is arranged to obtain data indicative of an angularvelocity of the cutting disc 105, and to detect a kickback conditionbased on a decrease in angular velocity, and to control anelectromagnetic braking of the electric motor 130 in response todetecting a kickback condition. The control unit 110 is arranged todetermine an angular acceleration associated with the electric motor130, and to detect the kickback condition based on a comparison betweenthe determined angular acceleration and a configurable detectionthreshold. This configurable threshold may also be variable, in thesense that it is adapted to the current operating conditions of thecut-off tool. The configurable threshold may either be a manuallyconfigurable threshold or an automatically configurable threshold, orboth a manually configurable threshold and an automatically configurablethreshold.

Herein, a configurable detection threshold is to be construed as adetection threshold which is not hard-coded during tool assembly, butwhich can be changed in one or more ways after the tool has left thefactory.

The detection threshold may be configured either manually, or independence of some other machine-dependent parameter, such as a cuttingdisc specification. The detection threshold may also be arranged to beconfigured over wireless or wired link from a remote unit, such as amobile device or a remote server.

Ideal detection thresholds can be arrived at through testing or bycomputer simulation. For instance, the detection threshold can beconfigured at an angular acceleration between 5000 rad/s² and 35000rad/s², and preferably between 10000 rad/s² and 30000 rad/s².

If the tool diameter of the rotatable cutting disc 105 is about 10inches, such as between 9 and 11 inches, then the detection thresholdcan be configured at an angular acceleration between 5000 rad/s² and35000 rad/s², and preferably between 20000 rad/s² and 35000 rad/s².

If the tool diameter of the rotatable cutting disc 105 is about 12inches, such as between 11 and 13 inches, then the detection thresholdcan be configured at an angular acceleration between 25000 rad/s² and35000 rad/s², and preferably about 29000 rad/s².

If the tool diameter of the rotatable cutting disc 105 is about 14inches, such as between 13 and 15 inches, then the detection thresholdcan be configured at an angular acceleration between 25000 rad/s² and35000 rad/s², and preferably about 29000 rad/s².

If the tool diameter of the rotatable cutting disc 105 is about 16inches, such as between 15 and 17 inches, then the detection thresholdcan be configured at an angular acceleration between 15000 rad/s² and25000 rad/s², and preferably about 20000 rad/s².

Generally, the ideal detection threshold decreases with increased toolinertia, such that a large diameter tool has a smaller detectionthreshold compared to a small diameter tool.

The control unit is optionally arranged to prevent operation of the toolduring a time window following detection of the kickback condition. Thisis because some tool components may become hot during the brakingoperation and needs to be cooled down before the tool can be used again.The time window can be between 5-30 seconds long, and preferably about25 seconds. Of course, it is desired to reduce this time window as muchas possible, since it can be a nuisance to an operator experiencingrepeated kickback conditions. Consequently, it may be an advantage toset the time window in dependence of a tool inertia. In this case largeinertia tools can have longer time windows compared to smaller inertiatools. The time window can also be configured in dependence of atemperature of the electric motor and/or energy dissipating module. Inthis case, the time window can be reduced in case the tool temperatures,and the motor temperature in particular, is relatively low, while thetime window can be prolonged in case the temperature or temperatures arehigh. In fact, the time window can be arranged to be terminated by atemperature threshold instead of a time limit on the window.

According to other aspects, the control unit can be arranged to suppressdetection of a kickback condition during a time period following startof the electric motor. This may reduce the number of false positives,i.e., kickback detections as a result of disturbances during tool start,where the motor currents may fluctuate some. The control unit may alsobe arranged to suppress detection of a kickback condition unless thetool engages an object to be cut. This can be detected via the motorcurrents since the motor experiences a change in load as he tool engagesthe object to be cut. This can also be detected by some other sensorarranged on the tool, like a strain gauge configured in connection tothe arm holding the cutting disc, or a linear displacement sensorconnected in-between the cutting disc 105 and the tool body, forinstance between the support arm 150 and the machine body 101.

According to some aspects, the control unit 110 is arranged to detectthe kickback condition also based on an angular velocity associated withthe electric motor 130 by conditioning kickback detection based on aminimum angular velocity. This angular velocity may, e.g., be theestimated rotor speed which results from the first differentiator 510,perhaps with some additional filtering applied. The conditioning may,e.g., comprise requiring a certain minimum initial velocity in order todetect a kickback event. The rationale for this conditioning being thata severe kickback event normally does not take place at low rotationalcutting disc velocities. Also, the estimate of rotor acceleration may beassociated with large errors during a start-up phase when the cuttingdisc is accelerated from stand-still or from a low velocity. The minimumangular velocity may be on the order of 400-800 rad/s, such as about 600rad/s.

Consequently, there is disclosed a hand-held electrically poweredcut-off tool 100 for cutting concrete and stone by a rotatable cuttingdisc 105. The cut-off tool 100 comprises an electric motor 130 arrangedto be controlled by a control unit 110 via a motor control interface120. The control unit 110 is arranged to obtain data indicative of anangular velocity of the cutting disc 105, and to detect a kickbackcondition based on a decrease in angular velocity. The control unit 110is also arranged to determine an angular acceleration associated withthe electric motor 130, and to detect the kickback condition based on acomparison between the determined angular acceleration and a detectionthreshold if the angular velocity of the cutting disc 105 is above avelocity threshold. This way false detections can be avoided. Here as inmost versions of the proposed concept, the detection threshold can be avariable detection threshold, e.g., one that is manually configurable orautomatically determined in dependence of an operating condition or atype of cutting disc used.

There is also disclosed a hand-held electrically powered cut-off tool100 for cutting concrete and stone by a rotatable cutting disc 105, thecut-off tool 100 comprising an electric motor 130 arranged to becontrolled by a control unit 110 via a motor control interface 120,wherein the control unit 110 is arranged to obtain data indicative of anangular velocity of the cutting disc 105, and to detect a kickbackcondition based on a decrease in angular velocity, wherein the controlunit 110 is arranged to determine an angular acceleration associatedwith the electric motor 130, and to detect the kickback condition basedon a comparison between the determined angular acceleration and adetection threshold, where the variable detection threshold is arrangedto be determined in dependence of one or more operating conditions ofthe tool 100 and/or in dependence of a configuration input signal. Theoperating conditions may, e.g., comprise tool inertia, tool type, toolangular velocity, or if the tool is engaging the object to be cut ornot. The configuration input signal may be a signal obtained from anoperator, or a configuration signal received as part of a configurationfile, such as a configuration file received from a wireless device, aremote server, or other configuration entity. According to some aspects,the possibility to vary the variable detection threshold can be limitedto be within some pre-determined range, in order to prevent inactivationor malfunction of the kickback detection system.

Thus, the configuration input signal may be generated by an operatorusing some form of input means, like an input display device on themachine body 101, or an input device arranged remote from the machine,such as a smartphone or other wireless device. The operator may then,e.g., select from a number of predefined cutting disc types (dimension,weight, etc.). The hand-held electrically powered cut-off tool 100 mayfurthermore be arranged to detect when a new cutting disc has beenassembled on the support arm 150, and prompt the user for a newconfiguration before allowing operation, or simply trigger anotification with a request to configure the tool.

There is furthermore disclosed herein a hand-held electrically poweredcut-off tool 100 for cutting concrete and stone by a rotatable cuttingdisc 105. The cut-off tool 100 comprises an electric motor 130 arrangedto be controlled by a control unit 110 via a motor control interface 120as discussed above. The control unit 110 is arranged to obtain dataindicative of an angular velocity of the cutting disc 105, and to detecta kickback condition based on a decrease in angular velocity. Thecontrol unit 110 is also arranged to determine an angular accelerationassociated with the electric motor 130, and to detect the kickbackcondition based on a comparison between the determined angularacceleration and a detection threshold selectable from at least twodifferent threshold values. Thus, it is appreciated that the detectionthreshold is variable in the sense that is can be selected from a set ofpre-configured thresholds. For instance, the detection threshold can beselected from the set in dependence of the current cutting disc that ismounted onto the machine. Different cutting discs are associated withdifferent amounts of inertia when in use, and therefore ideally are tohave different detection thresholds for detecting kickback condition.Generally, a heavier disc changes velocity more slow compared to alighter weight cutting disc. Also, lighter weights cutting discs tend toexhibit more fluctuations in the velocity, requiring more filtering,i.e., a smaller bandwidth filter. Generally, the detection threshold canbe configured at an angular acceleration between 5000 rad/s² and 35000rad/s², and preferably between 10000 rad/s² and 30000 rad/s², and morepreferably between 20000 rad/s² and 30000 rad/s², depending on the typeof cutting disc used and the operating environment.

The detection threshold may according to one example be manuallyconfigurable. In this case configuration data is manually input to theevaluation module 530, where it is used to determine the detectionthreshold.

According to other aspects, the control unit 110 is arranged to obtaindata indicative of a tool diameter or tool inertia of the rotatablecutting disc 105, and to adjust the detection threshold based on thedata indicative of tool diameter or tool inertia. This data may, forinstance, be obtained by the control unit 110 as manual input orautomatically. Manually input data may, e.g., be input by an operatorvia manual input means such as a touch screen or keypad. The controlunit may also be arranged to obtain the data indicative of the tooldiameter or tool inertia from a source different from a manual input,such as e.g., via wireless link from a remote server or otherconfiguration device, such as a smartphone. Other methods forautomatically determining the data indicative of the tool diameter ortool inertia without need for manual input will be provided below.

Herein, data indicative of a tool diameter also indicates an amount ofinertia associated with the cutting disc 105. The larger the inertia,the more kinetic energy must be handled during a kickback event. Thismeans that different types of tools having different weights anddifferent tool diameters may require different detection threshold inorder for the kickback mitigation function to provide the desiredperformance.

The control unit 110 can also be arranged to obtain the data indicativeof the tool diameter or tool inertia based on a calculated or estimatedtool inertia. This tool inertia can, for instance, be determined basedon a current drawn by the electric motor during acceleration from astandstill or low velocity condition, i.e., it can be determined fromthe current measurement on the control interface 120 in FIG. 4 duringacceleration of the cutting disc 105. The processor 410 may comprise alook-up table or the like which allows for translating between estimatedtool inertia values and suitable detection thresholds. Alternatively, ananalytic function can be used to determine suitable detection thresholdsfrom the estimated tool inertia. Also, low-pass filtering operationsused to determine, e.g., rotor acceleration values, can be configured independence of such estimated tool inertia. This is because a highinertia tool is expected to change rotor velocity somewhat more slowlywhich may warrant a reduced filtering bandwidth to suppress more noisewhen estimating, e.g., rotor speed and rotor acceleration.

The cut-off tool may furthermore comprise a radio frequencyidentification (RFID) reader. In this case the control unit 110 can bearranged to obtain the data indicative of the tool diameter or toolinertia from an RFID device embedded into or otherwise arranged inconnection to the cutting disc 105 via the RFID reader. According toanother example, the tool data may be stored on a remote server. If thecut-off tool comprises a radio transceiver, the control unit 110 can bearranged to obtain data indicative of the tool diameter or tool inertiafrom a remote server via the radio transceiver.

The cut-off tool may also comprise other means for identifying, e.g.,the type of tool. Such means for identification may comprise opticallyreadable tags such as QR-codes, or punch-card like symbols which can beread optically and used to index a database on, e.g., the remote server,to obtain the data indicative of the tool diameter or tool inertia.

Of course, the data indicative of the tool diameter or tool inertia canalso be manually input to the control unit 110.

The rotational data can also be obtained from an external sensor, suchas a Hall effect sensor arranged to measure a rotational velocity of ashaft in the drive arrangement, such as a motor shaft or a pulley shaft,or even the shaft of the cutting tool 105 itself. This rotational datacan be used in combination with the rotor angle estimate data obtaineddirectly from the electric motor, or it can be used in place of thisdata as an alternative source of information by which the kickbackdetection can be performed.

With reference to FIGS. 6-8 , the cut-off tool 100 may comprise a beltdrive arrangement in the support arm 150 configured to provide a driveratio which reduces the rotational speed of the electric motor driveshaft down to a speed suitable for processing concrete, e.g., about3500-4500 revolutions per minute (rpm). This is an advantage sinceelectric motors operating at reduced engine speeds are more costly andoften also weighs more than standard motors operating around 9000-10000rpm. Such gear ratios necessitate using a smaller pulley at the motordrive shaft to drive a larger pulley connected to the work tool.However, if the larger pulley is co-axially attached directly to therotatable work tool, then the attainable cutting depth may be reduced bythe large belt pulley.

The drive arrangement illustrated in FIGS. 6-8 is based on a combinationof a drive belt portion and a gear transmission portion. The belt driveportion comprises a first pulley 610 and a second pulley 630 with adrive belt 620 in between. To reduce blade speed with respect to arotational speed of the first pulley, the second pulley has a largerpitch diameter than the first pulley. This drive ratio increases torqueand reduces speed making the rotatable work tool suitable for drycutting operation. The drive arrangement also comprises a geartransmission portion as shown in FIG. 8 . The gear transmission portioncomprises a first gearwheel 810 and a second gearwheel 820. The firstgearwheel 810 is co-axially connected to the second pulley 630 and thesecond gearwheel 820 is arranged to be co-axially connected to therotatable work tool 105. Thus, as the first pulley 610 is rotated, thebelt 620 drives the second pulley 630 in the same direction of rotation.The second pulley, being co-axially connected to the first gearwheel810, then drives the first gearwheel in the same direction of rotationas the first pulley 610. The first gearwheel 810 is radially connectedto the second gearwheel 820, and therefore drives the second gearwheelin an opposite direction of rotation. Thus, the direction of rotation ofthe first pulley and the direction of rotation R of the work tool 105are opposite to each other. This is not a problem when using an electricmotor as a power source, which can be configured to rotate in anydirection. Thus, the disclosed drive arrangements are especially suitedfor use with electric motors.

The gear transmission portion is dimensioned to support a braking actionby the electric motor to stop rotation by the rotatable work tool from arotation velocity of about 50 m/sec in 5 ms, for a given belt dimension.Effectively this means that, due to the gear transmission portion, thepower source can be parameterized more aggressively for a brakingoperation, without placing undue requirements on the belt drive portion,and the belt in particular, which is an advantage.

According to some aspects, a ratio of the first gearwheel 810 pitchdiameter and the second gearwheel 820 pitch diameter is between 0.4 and0.6, and preferably 0.56. According to an example, the first gearwheel810 has a pitch diameter between 20 and 35 mm, preferably 28 mm, and thesecond gearwheel 820 has a pitch diameter between 40 and 60 mm,preferably 50 mm. Regarding the belt drive portion, the first pulley 610may be associated with a pitch diameter between 30 and 40 mm, preferably35.4 mm, and the second pulley 630 may be associated with a pitchdiameter between 60 mm and 70 mm, preferably 64.85 mm. According toaspects, a ratio between a pitch diameter of the first pulley and apitch diameter of the second pulley is between 0.4 and 0.6, andpreferably about 0.55. Various types of drive belts can be used in thebelt drive portion, such as a v-belt or a toothed belt.

Thus, according to some aspects, the cut-off tool 100 comprises asupport arm 150, wherein the rotatable cutting disc 105 is arranged tobe driven by the electric motor 130 via a belt drive 610, 620, 630 and ageared transmission 810, 820.

There is also disclosed herein a hand-held electrically powered cut-offtool 100 for cutting concrete and stone by a rotatable cutting disc 105.The cut-off tool 100 comprises an electric motor 130 arranged to becontrolled by a control unit 110 via a motor control interface 120 asillustrated in, e.g., FIG. 2 and FIG. 3 . The control unit 110 isarranged to determine an angular position of a rotor of the electricmotor 130, or, equivalently, of the electric motor shaft, based on acurrent over the control interface 120, and to obtain data indicative ofan angular velocity of the cutting disc 105 based on a rate of change ofthe angular position of the rotor (or shaft) over time. The control unit110 is arranged to detect a kickback condition based on a decrease inangular velocity of the cutting disc 105, and to control electromagneticbraking of the electric motor 130 in response to detecting a kickbackcondition.

FIG. 9 is a flow chart illustrating methods, there is illustrated amethod in a control unit 110 for mitigating kickback in a hand-heldelectrically powered cut-off tool 100 arranged for cutting concrete andstone by a rotatable cutting disc 105, wherein the cut-off tool 100comprises an electric motor 130 arranged to be controlled by the controlunit 110 via a motor control interface 120. The method comprises

-   -   obtaining S1 data indicative of an angular velocity of the        cutting disc 105,    -   detecting S2 a kickback condition based on a decrease in angular        velocity, and    -   electromagnetically braking S3 the electric motor 130 in        response to detecting a kickback condition.

Optionally, the method also comprises actively regulating S4 an energyouttake from the electric motor 130 over the control interface 120during the electromagnetic braking.

FIG. 10 schematically illustrates, in terms of a number of functionalunits, the general components of a control unit 110. Processingcircuitry 1010 is provided using any combination of one or more of asuitable central processing unit CPU, multiprocessor, microcontroller,digital signal processor DSP, etc., capable of executing softwareinstructions stored in a computer program product, e.g. in the form of astorage medium 1030. The processing circuitry 1010 may further beprovided as at least one application specific integrated circuit ASIC,or field programmable gate array FPGA.

Particularly, the processing circuitry 1010 is configured to cause thedevice 180 to perform a set of operations, or steps, such as the methodsdiscussed in connection to FIG. 9 and the discussions above. Forexample, the storage medium 1030 may store the set of operations, andthe processing circuitry 1010 may be configured to retrieve the set ofoperations from the storage medium 1030 to cause the device to performthe set of operations. The set of operations may be provided as a set ofexecutable instructions. Thus, the processing circuitry 1010 is therebyarranged to execute methods as herein disclosed.

The storage medium 1030 may also comprise persistent storage, which, forexample, can be any single one or combination of magnetic memory,optical memory, solid state memory or even remotely mounted memory.

The device 110 may further comprise an interface 1020 for communicationswith at least one external device. As such the interface 1020 maycomprise one or more transmitters and receivers, comprising analogue anddigital components and a suitable number of ports for wireline orwireless communication.

The processing circuitry 1010 controls the general operation of thecontrol unit 110, e.g., by sending data and control signals to theinterface 1020 and the storage medium 1030, by receiving data andreports from the interface 1020, and by retrieving data and instructionsfrom the storage medium 1030.

FIG. 11 illustrates a computer readable medium 1110 carrying a computerprogram comprising program code means 1120 for performing the methodsillustrated in FIG. 9 , when said program product is run on a computer.The computer readable medium and the code means may together form acomputer program product 1100.

1. A hand-held electrically powered cut-off tool for cutting concreteand stone by a rotatable cutting disc, the cut-off tool comprising anelectric motor arranged to be controlled by a control unit via a motorcontrol interface, wherein the control unit is arranged to obtain dataindicative of an angular velocity of the cutting disc, and to detect akickback condition based on a decrease in angular velocity, wherein thecontrol unit is arranged to control an electromagnetic braking of theelectric motor in response to detecting a kickback condition, whereinthe control unit is arranged to determine an angular accelerationassociated with the electric motor, and to detect the kickback conditionbased on a comparison between the determined angular acceleration and aconfigurable detection threshold.
 2. The hand-held electrically poweredcut-off tool according to claim 1, wherein the control unit is arrangedto detect the kickback condition also based on an angular velocityassociated with the electric motor by conditioning kickback detectionbased on a minimum angular velocity.
 3. The hand-held electricallypowered cut-off tool according to claim 2, wherein the minimum angularvelocity is between 400 rad/s and 800 rad/s.
 4. The hand-heldelectrically powered cut-off tool according to claim 1, wherein thedetection threshold is manually configurable and/or remotelyconfigurable.
 5. The hand-held electrically powered cut-off toolaccording to claim 1, wherein the control unit is arranged to obtaindata indicative of a tool diameter or indicative of a tool inertia ofthe rotatable cutting disc, and to adjust the configurable detectionthreshold based on the data indicative of tool diameter or tool inertia.6. The hand-held electrically powered cut-off tool according to claim 5,wherein the control unit is arranged to obtain the data indicative ofthe tool diameter or tool inertia from manual input.
 7. The hand-heldelectrically powered cut-off tool according to claim 5, wherein thecontrol unit is arranged to obtain the data indicative of the tooldiameter or tool inertia from a source different than a manual input. 8.The hand-held electrically powered cut-off tool according to claim 5,wherein the control unit is arranged to obtain the data indicative ofthe tool diameter or tool inertia based on a calculated or estimatedtool inertia, wherein the tool inertia is arranged to be determinedbased on a current drawn by the electric motor during acceleration. 9.The hand-held electrically powered cut-off tool according to claim 5,wherein the cut-off tool comprises a radio frequency identification,RFID, reader, and wherein the control unit is arranged to obtain thedata indicative of the tool diameter or tool inertia from an RFID deviceembedded into the cutting disc via the RFID reader.
 10. The hand-heldelectrically powered cut-off tool according to claim 5, wherein thecut-off tool comprises means for detecting tool identification data, ID,from an optically readable tag arranged on the tool or on a toolpackaging associated with the tool, and to obtain the data indicative ofthe tool diameter or tool inertia based on the tool ID.
 11. Thehand-held electrically powered cut-off tool according to claim 1,wherein the detection threshold is configured at an angular accelerationbetween 5000 rad/s² and 35000 rad/s².
 12. The hand-held electricallypowered cut-off tool according to claim 1, wherein the tool diameter ofthe rotatable cutting disc is between 9-11 inches, wherein the detectionthreshold is configured at an angular acceleration between 5000 rad/s²and 35000 rad/s².
 13. The hand-held electrically powered cut-off toolaccording to claim 1, wherein the tool diameter of the rotatable cuttingdisc is between 11-13 inches, wherein the detection threshold isconfigured at an angular acceleration between 25000 rad/s² and 35000rad/s².
 14. The hand-held electrically powered cut-off tool according toclaim 1, wherein the tool diameter of the rotatable cutting disc isbetween 13-15 inches, wherein the detection threshold is configured atan angular acceleration between 25000 rad/s² and 35000 rad/s².
 15. Thehand-held electrically powered cut-off tool according to claim 1,wherein the tool diameter of the rotatable cutting disc is between 15-17inches, wherein the detection threshold is configured at an angularacceleration between 15000 rad/s² and 25000 rad/s².
 16. The hand-heldelectrically powered cut-off tool according to claim 1, wherein thecontrol unit is arranged to detect the kickback condition based on acomparison between the determined angular acceleration and a detectionthreshold selectable from at least two different threshold values. 17.The hand-held electrically powered cut-off tool according to claim 1,wherein the configurable detection threshold is an automaticallyconfigurable threshold.
 18. A hand-held electrically powered cut-offtool for cutting concrete and stone by a rotatable cutting disc, thecut-off tool comprising an electric motor arranged to be controlled by acontrol unit via a motor control interface, wherein the control unit isarranged to obtain data indicative of an angular velocity of the cuttingdisc, and to detect a kickback condition based on a decrease in angularvelocity, wherein the control unit is arranged to determine an angularacceleration associated with the electric motor, and to detect thekickback condition based on a comparison between the determined angularacceleration and a variable detection threshold, where the variabledetection threshold is arranged to be determined in dependence of one ormore operating conditions of the tool and/or in dependence of aconfiguration input signal.
 19. A hand-held electrically powered cut-offtool for cutting concrete and stone by a rotatable cutting disc, thecut-off tool comprising an electric motor arranged to be controlled by acontrol unit via a motor control interface, wherein the control unit isarranged to obtain data indicative of an angular velocity of the cuttingdisc, and to detect a kickback condition based on a decrease in angularvelocity, wherein the control unit is arranged to determine an angularacceleration associated with the electric motor, and to detect thekickback condition based on a comparison between the determined angularacceleration and a detection threshold selectable from at least twodifferent threshold values.